In order to implement highly integrated electrical circuits with small structure dimensions, lithographic structuring methods are normally used. In this case, a radiation-sensitive photoresist layer is applied to the surface of a semiconductor substrate wafer to be structured and is exposed through an exposure mask with the aid of electromagnetic radiation. In what is known as photolithography, ultraviolet light is used in the process. During the exposure procedure, a lithographic structure arranged on the exposure mask is projected onto the photoresist layer and, with the aid of subsequent etching processes, is transferred into the photoresist layer and the semiconductor layer located underneath.
Known exposure masks comprise a transparent carrier plate, designated a reticle, on which the lithographic structure is arranged as a structured chromium layer. During the exposure process, this structure is projected onto the photoresist layer of the substrate wafer either on a scale of 1:1 or, as in the reducing projection exposure which is familiar nowadays, in a ratio of, for example, 5:1 with the aid of an optical lens system. The reducing projection exposure permits the production of very small structures.
In order to avoid projection errors, which can be caused by particles accidentally deposited on the lithographic structure, a thin transparent membrane, designated a pellicle, is used which, with the aid of a frame arranged on the reticle, is clamped on at a defined distance from the lithographic structure, so that the lithographic structure is sealed. The encapsulating structure of the exposure mask ensures that individual particles from the surroundings of the exposure mask could not be deposited directly on the lithographic structure at the optical focus and could not then disrupt the exposure process.
The resolving power of an exposure method, and therefore the achievable size of the semiconductor structures, is limited by the wavelength of the radiation used. In the course of miniaturization of these structures, constantly required in the semiconductor industry, smaller and smaller exposure wavelengths are being used. Because of a wavelength-dependent transparency of the components of the exposure mask, novel materials are also used for smaller exposure wavelengths.
For an exposure wavelength of 436 nm, the reticle consists of normal glass and, for wavelengths of 365 nm, 248 nm and 193 nm, and for the wavelength of 157 nm to be used in the future, the reticle consists of quartz glass.
For wavelengths of 436 nm and 365 nm, the pellicle is a cellulose nitrate film and, for wavelengths of 248 nm and 193 nm, the pellicle is a fluoropolymer film. These pellicles consisting of organic materials are also designated a “soft pellicle”. However, since a fluoropolymer film exhibits photochemical aging processes at an exposure wavelength of 193 nm, it is more beneficial, at this wavelength and in particular at the still higher-energy wavelength of 157 nm, to use a thin quartz glass sheet as a pellicle. A pellicle of this type is also called a “hard pellicle”.
The frame for connecting reticle and pellicle consists of anodized and dyed aluminum at the exposure wavelengths of 436 nm to 193 nm used hitherto. However, for the future exposure wavelength of 157 nm, it is more beneficial to form a frame likewise of quartz glass, in order to avoid projection errors or damage to the pellicle because of sagging in the event of a small temperature change, because of the different thermal expansion coefficients of quartz glass and aluminum.
As an adhesive agent for connecting the reticle, the frame and the pellicle, in the case of exposure masks for the exposure wavelengths of 436 nm to 193 nm used hitherto, organic adhesives, predominantly acrylates are used. However, in the case of these organic compounds, there is the problem that they tend to gas out and, as a result, in spite of the sealing achieved with the aid of the frame and the pellicle, particles can be deposited on the lithographic structure on the reticle. For example, it is possible to imagine that, at higher temperatures, organic molecules will gas out from the adhesive, will be deposited at thermodynamically favored points on the lithographic structure and crystals will grow as a result. Amongst other things, it is also possible that constituents of the air will form crystals on the lithographic structure with gassing of the adhesive.
In addition, it is disadvantageous that these radiation-induced chemical reactions increase, the smaller the exposure wavelength used is, since the radiation energy increases as the wavelength is reduced. In parallel with this, the mask structures are becoming smaller and smaller in the course of the miniaturization of the semiconductor structures and, as a result, are becoming more and more susceptible to deposits on the lithographic structure.